Plaster of Paris is made from gypsum rock that changes from a solid form into a powder and back to a solid quite easily by adding water. It is a popular medium for arts and craft projects requiring molds.
Cover your work area with plastic or newspapers for easy clean-up and wear old clothes. Casting plaster gets messy. Have all your tools close at hand.
Choose your molds. Candy molds or molds made specifically for plaster casting are fine. Plastic molds work well. You'll need to start with clean molds with no residue from other castings. Make sure the mold is dry, as water slows down the setting process. If you've never worked with plaster of Paris, start with a smaller mold so you can perfect the process.
Follow mixing directions that came with your plaster of Paris. Using cool water will keep the plaster from hardening too quickly. A good rule of thumb for proper consistency is to make the plaster cake-batter thick. If the mix is too thick, it may start to harden before you're ready. If it's too thin, it will take a lot longer to dry, and your finished product may come out powdery.
Pour the plaster of Paris into your mold slowly so the plaster gets into all the details and crevices of the mold. Gently tap the mold on a hard surface to release any air bubbles that may be trapped at the bottom of the mold. Wait for the plaster to dry. This can take anywhere from 10 minutes to several hours, depending on how large your mold is.
Remove your plaster object by loosening the edges of the plaster from the mold. This process is much like removing a cake from a cake pan. Don't force the finished plaster from the mold. If the item feels stuck, gently twist the mold until the plaster object works its way out.
Use a knife to gently trim away excess plaster from the edges. If necessary, use a fine-grain sandpaper to further smooth the rough spots.
Make molds out of plaster of Paris. Mix the plaster as directed by the manufacturer. Pour the plaster into a watertight container, such as a plastic box or a bowl. If you're making a mold of a porous object, coat it with oil or petroleum jelly. Place the object onto the plaster. Push down slightly so the plaster gets into crevices and details. Wait for the plaster to dry, then gently remove the object.
After your mold is set and completely dry, use oil or petroleum jelly to create a water-tight space between your pouring agent and a plaster of Paris mold. This will help keep your finished product and your plaster mold from sticking together. Plaster of Paris is water-soluble. Always keep a barrier between your mold and other water-based products, or you may end up chipping the mold off your item.
- Plaster of Paris heats up when curing. Be cautious when making hand and foot imprints.
- Use fresh plaster of Paris to ensure proper consistency when mixing.
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